Photoelectric sound reproduction



Jan. 22, 1952 l.. E. CLARK 2,582,964

PHOTOELECTRIC SOUND REPRODUCTION Filed March 8, 1949 E Mcm/m arf 6745Pff-$501915' ZZV m 102 M e Patented Jan. 22, 1952' l PHo'roELEc'rmcsoUND REPRODUCTION Lauriston E. Clark, Los Angeles, Calif., assignor toTechnicolor Motion Picture Corporation, Hollywood, Calif., a corporationof Maine Application March 8, 1949, Serial No. 80,153

claims. l

It is desirable in the field of color cinematography to record sound byway of colored tracks formed by conventional methods similar to thoseused for printing the picture records which the sound tracks areintended to accompany. The coloring matter of such sound tracks mayconsist of mechanically or chemically applied colored pigment, or of thematerial in which the pictures are preferably formed at the present timenamely dyes mechanically transferred by imbibition printing from arecord matrix to a blank film, or dyes photographically developed withinthe emulsion by now conventional methods. or dyes initially incorporatedin the emulsion and subsequently partly destroyed to form a record.

.- Sound tracks formed in coloring matter which is by necessityprimarily selected on the basis of suitability for picture recording,have certain disadvantages. One of these is that such tracks cannot beproperly reproduced with the aid of the cesium-oxygen-silver photocellscustomarily used in motion picture theatre equipment, because the dyesin question are highly transmissive in the infrared spectral range towhich these cells are sensitive, so that it is dimcult to obtain signalsthat are initially strongly accentuated by differential absorption ofcell affecting light. Photocells which are not sensitive to the infraredrange, or generally speaking photocells which are mainly sensitive to aspectral range within which the dye in question strongly absorbs, haveheretofore not been used because of the lack of correspondence ofsensitivity and transmission ranges of photocells and dyes respectively.

The problem is further complicated by the requirement that, if at allpossible. only a single dye should be used for the sound track in orderto avoid complications which obviate the advantages inherent in usingdye sound tracks. This requirement excludes matching of track andphototube by way of modifying the dye absorption. For example the singlesubtractive dye of magenta hue which transmits blue and red light and isused for reproducing the green color aspect picture record, has anabsorption range which fairly well agrees with the sensitivity range ofa now available cesium-antimony phototube (essentially insensitive toinfrared light) when illuminated by tungsten light. In actual practicehowever this combination has not proved to be satisfactory. The qualityof reproduction by such an arrangement is below that of a fine grainedsilver sound track, particularly 'with regard to the signal-to-nolseratio which depends to a considerable degree upon the effective densityof the opaque sections of the track which is in the case of a soundtrack and photocell combination of this type lower than desirable. Aswill be explained more in detail below' when describing a specificembodiment of the invention utilizing a cesium-antimony cell with amagenta sound track. the absorption range ofv a colored sound track isapt to be narrower than the sensitivity range of the phototube used forscanning that track. This causes loss of effective density due to thefact that the track passes light of the spectral range adjacent to theprimary effective range of the tube which is in effect analogous to theabove-mentioned drawback resulting from the infrared transmission ofmost dyes available for colored sound tracks.

This discrepancy between dye (or other coloring matter) absorption andphototube sensitivity cannot be corrected by modifying the dye, forreasons indicated above nor can it be corrected by interposing aconventional lter between light source and tube with the idea ofeliminating the undesirable border absorption ranges of the dye. becausethe loss of total output would be so great as to require additionalamplification in each theatre sound equipment whether or not primarilyintended for use with such tracks, which is impractical for obviousreasons. It is wellknown that the sensitivity of a photocell can beincreased by increasing the pressure of the inert gas filling the tube;this however is impractical as applied to the above suggestedassociation of the tube with a conventional filter, because theincreased pressure renders the cell unstable when it is over-illuminatedas for example by the operator using a trouble lamp for examining theequipment while the photocell polarizing voltage is applied. The cellwould pass excessive current during such examinations which soon resultsin destruction of the photosensitive surface.

It is the principal object of the present invention to provide a methodof reproducing colored sound tracks with a quality comparable to thatfurnished by fine grained silver tracks with equipment that is ofstandard type so far as the practical operation of the sound reproducingcomponents is concerned, that is without in any way changing the shapeof the phototube, without changing the illuminant, and without, addingcomponents such as a separate filter or additional amplifying equipmentnot used in standard apparatus. Another object is to provide a phototubeof conventional configuration which closely adjacent the firstrangefsuch as the n blue and yellow border ranges), by enveloping thecathode in a vessel which absorbs these adjacent or border ranges, andby projecting the track on the cathode with light containing theabove-mentioned ranges (such as tungsten light) whereby the tube can besafely handled when intensely illuminated. In a preferred embodiment thetube is filled with an inert gas at comparatively high pressure for thepurpose of increasing the sensitivity of the cathode, to render thephototube sensitivity approximately equal to that of a tube having asimilar cathode in an essentially clear vessel filled with gas at thestandard pressure, thus preserving the sensitivity of such tubes withclear vessel and standard pressure. In another aspect. the inventionprovides a gas filled phototube for use in sound reproduction from acolored sound track (for. example in green absorbing dye) which tubecomprises a cathode sensitive to a light range absorbed by the soundtrack and in addition to adjacent ranges (for example towards the blueand yellow) this cathode being coated with appropriate material (forexample a cesium-antimony alloy if the dye is a-bsorptive of the greenspectral range). and the envelope of the tube being colored to absorbthe adjacent ranges being for example of glass containing substances(for example suitable metal compounds) rendering` it absorptive in thismanner or being covered with a filter layer absorptive of these adjacentranges (for example a lacquer layer containing suitable coloringmatter); in a preferred embodiment the envelope is filled with an inertgas (such as argon if a cesium-antimony cathode is used) at a pressurerendering the tube approximately as sensitive as an otherwise similartube having an essentially clear vessel filled with gas of lowerpressure.

These and other objects, aspects. and features of practical constructionwill appear in the following description of two typical practicalembodiments illustrating the novel characteristics of my invention. Thisdescription refers to a drawing in which Fig. 1 is a diagramillustrating the phototube sensitivity and dye absorptioncharacteristics present in a system according to the invention;

Fig. 2 is a diagrammatic elevation, with part of the envelope brokenaway. of a-phototube according to the invention; and

Fig. 3 is a diagram illustrating a sound detecting arrangement accordingto the invention.

In Fig. l. A is the sensitivitycurve of a cesiumantimony phototube ofthe type commercially known as 1F37 cell. Curve B indicates the lightabsorption of a conventional magenta (minus green) dye used for makingmotion picture positives by dye imbibition from wash-.off gelatinreliefs. It will be noted that the dye transmits considerable light inthe wave length ranges 400 to 460 and 600 to 700 millimicrons, adjacentthe region of maximum sensitivity of the phototube cathode atapproximately 520 millimicrons. This would ordinarily lower the capacityof the sound track in its high density portion thus causing impairmentof the quality of sound reproduction particularly as regards thesignal-to-noise ratio. This loss of effective density is caused by lightof wave lengths 40G-600 and 600-700 passing through the magenta dye withrelative ease. These wave lengths are emitted by the tungsten filamentlamps customarily used for sound track scanning. y

In accordance with the invention the glass envelope of the phototube ismade of green glass or covered with a green filter layer of the propercharacteristics in order to modify the effective sensitivity of the tubeas indicated at C of Fig. l. Curve C indicates that a phototube of thisdesign responds practically only to a narrow region around theabove-mentioned maximum cathode sensitivity.

If the glass envelope itself is used as a filter it may be made of thesoft glass customarily used for machine manufacture of bulbs of thistype, containing about 14 per cent NazO and 5 to 6 per cent CaO, towhich have been added, according to the invention, copper and chromiumoxides in amounts up to about 1 per cent. This admixture limits theeffective transmission of the glass essentially to the region from 520to 540 millimicrons, with a fairly sharp cut on both sides of thisregion, thus filtering out the light transmitted by the magenta soundtrack dye in the above-mentioned ranges adjacent to the region ofmaximum effectivity.

If it is desired to coat the phototube instead of using a colored glassenvelope, the tube is sprayed with or dipped into a nitro celluloselacquer solution containing for example l per cent each by weight of thedyes Guinea Green B (C1666) and Quinoline Yellow Spirit Soluble (C1800).

Fig. 2 indicates this construction `of the envelope I0 of a tube P andalso tie construction of the cathode Il, so far as essential for purposeof the present invention, namely a' cathode coated with an alloy ofantimony and cesium. A method of producing such an alloy layer is forexample described in U. S. Patent No. 2,122,860, comprising the steps ofevaporating antimony on the effective portion of the cathode, treatingthis antimony layer with oxygen, and then eve;L orating cesium thereon.Subsequent heat treatment of the layers causes the metals to form alight sensitive alloy responding with electron emission principally toradiation with the blue and green regions of the spectrum. As mentionedabove, this sensitivity i's indicated at A of Fig. l which curveillustrates this sensitivity with regard to illumination by tungstenlight customarily used in motion picture sound detecting apparatus.

As already mentioned, the tube sensitivity is considerably decreased bythe filter so that, withfrom 0.1 to millimeters of mercury column, ascompared with less than 0.1 millimeter of conventional tubes of thistype. As an approximate rule, the sensitivity of a cell with integralfilter envelope according to the invention will be increased to thelevel of an otherwise similar tube with clear envelope,` that isaccording to present practice to a sensitivity not exceeding 150microamperes per lumen.

Fig. 3 indicates a conventional sound gate incorporating the presentinvention. The film F has a picture area p upon which picture records Rcan be printed for example by imbibition techniques. these picturerecords including a green absorbing magenta dye. The sound track s isprinted in the same magenta dye whose absorption is indicated at B ofFig. 1. The film F is threaded through a conventional sound gate G whichcomprises an optical system O-including a slit S, a tungsten lamp L anda phototube P of the type described above with reference to Fig. 2. f

It will now be evident that the method and arrangement according to theinvention correct the deficiency in phototube sensitivity due to theoverlap, adjacent to the regions of maximum sensitivity and absorption,of the sensitivity and transmission ranges of tube and sound trackrespectively. The sensitivity of the system as a whole can be restoredto the conventional level by increasing the gas pressure, and the tubeof the new system is permanently protected against excess illuminationto`the same extent as conventional units by the lter layer which hasthus the double function of correcting the phototube sensitivity and ofpreserving theoperativeness of the tube. It should be noted in thiscontext that the intrinsic sensitivity of the cesium-antimony surface ishigh and that the secondary emission has been kept low in theconventional photocell of this type by limiting the gas pressure toabout one-tenth of the permissible or otherwise preferable value, forthe purpose of protecting the cathode against excess illumination. Theinvention utilizes this fact that the gas pressure can be increased from8- to 12 times above the level of the normal tube if this protection isinstead provided by way of the filter envelope according to theinvention. Thus, the phototube of a system according to the invention isnot exposed to invjury by excessive current destroying the sensitivecathode surface upon strong illumination with a flash light or troublelamp, while it is suiiiciently sensitive without additionalampliiication equipment, in spite of the reduced transparency of itsenvelope.

It will be evident that thev principle upon which the above describedsystems is based can be applied to photocells and coloring matters whichhave specific sensitivity and transmission characteristics diierent fromthose above described, so long as these characteristics have the general'relation herein dealt with.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modiiicationsand equivalents which fall within the scope or the appended claims.

I claim:

1. A phototube for use in sound reproduction from a colored track whichabsorbs a selected intermediate range of the visible spectrum,comprising a cathode sensitive to said range and to two adjacent ranges,and an envelope for said cathode colored to absorb said adjacent ranges.

2. A gas filled phototube for use in sound reproduction from a coloredtrack which absorbs a selected intermediate range of the visiblespectrum, comprising a cathode sensitive to said range and to twoadjacent ranges, an envelope for said l cathode colored to absorb saidadjacent ranges, and within said envelope an inert gas at a pressurerendering the tube approximately as sensitive as an otherwise similartube of standard sensitivity having an essentially clear vessel andiilled with lgas at lower pressure.

3. A phototube for use in sound reproduction from a colored track ingreen absorbing dye comprising a cathode sensitive to the green rangeabsorbed by the dye and to adjacent ranges towards the blue and yellow,and an envelope for said cathode colored to absorb said adjacent blueand yellow ranges.

4. A gas filled phototube for use in sound reproduction from a coloredtrack comprising a.

cathode coated with a cesium-antimony alloy, a glass envelope for saidcathode containing cop per and chromium oxides rendering said envelopeabsorbent of blue and yellow light and within said envelope argon at apressure of from 0.1 to 10 mm. mercury column.

5. A gas filled phototube for use in sound re,

, production from a colored track comprising a cathode coated with acesium-antimony alloy, a glass envelope for said cathode covered with alayer of nitro cellulose lacquer containing Guinea Green B and QuinolineYellow Spirit Soluble, and within saidenvelope argon at a pressure offrom 0.1 to 10mm. mercury column.

Y LAURISTON E. CLARK.

REFERENCES CITED The `following references are oi.' record in the fileof this patent:

773,398 France 1---- Sept. 3, 1934

